The enzyme, carbonic anhydrase, has been detected at the light microscope level in osteoclasts by the labeled inhibitor technique and by the use of fluorescent antibodies. We plan to adapt these procedures for use at the electron microscope level to localize its site of action so that we may further understand the role of this enzyme in skeletal physiology. In addition we plan to manipulate calcium levels in the diet and administer parathyroid hormone (extract) to experimental animals (rabbits and chickens) to induce changes in osteoclast populations and activity. The osteoclasts will then be observed as described above.